CVE-2022-20933 Overview
A vulnerability exists in the Cisco AnyConnect VPN server of Cisco Meraki MX and Cisco Meraki Z3 Teleworker Gateway devices that could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition. The vulnerability stems from insufficient validation of client-supplied parameters during SSL VPN session establishment. By crafting and sending malicious requests to affected devices, an attacker can crash and restart the AnyConnect VPN server, terminating all established SSL VPN connections and forcing remote users to reconnect and re-authenticate. A sustained attack can prevent new SSL VPN connections from being established entirely.
Critical Impact
Unauthenticated remote attackers can disrupt enterprise VPN services, forcing mass disconnection of remote workers and potentially blocking all new VPN connections during sustained attacks.
Affected Products
- Cisco Meraki MX Series Security Appliances (MX64, MX64W, MX65, MX65W, MX67, MX67C, MX67CW, MX67W, MX68, MX68CW, MX68W, MX75, MX84, MX85, MX95, MX100, MX105, MX250, MX400, MX450, MX600)
- Cisco Meraki vMX Virtual Appliance
- Cisco Meraki Z3 and Z3C Teleworker Gateway devices
Discovery Timeline
- October 26, 2022 - CVE-2022-20933 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2022-20933
Vulnerability Analysis
This vulnerability affects the Cisco AnyConnect VPN server component running on Cisco Meraki MX security appliances and Z3 Teleworker Gateway devices. The flaw exists in the SSL VPN session establishment process, where the server fails to properly validate parameters supplied by connecting clients. When malformed parameters are submitted during the VPN handshake process, the server crashes and initiates a restart sequence. During this restart, all active SSL VPN sessions are terminated, requiring connected users to establish new connections and complete re-authentication.
The vulnerability can be exploited remotely over the network without requiring authentication, making it particularly dangerous for organizations relying on these devices for remote workforce connectivity. While the VPN server recovers automatically after the attack traffic stops, sustained attacks can effectively maintain a denial of service condition for the duration of the attack.
Root Cause
The root cause of CVE-2022-20933 is classified under CWE-234 (Failure to Handle Missing Parameter) and relates to improper input validation. The Cisco AnyConnect VPN server implementation in affected Meraki firmware does not adequately validate all client-supplied parameters during the SSL VPN session negotiation phase. When the server encounters unexpected, malformed, or missing parameter values, it fails to handle the error gracefully, resulting in a crash condition rather than safely rejecting the malformed request.
Attack Vector
The attack is executed remotely over the network against the SSL VPN service port on vulnerable Meraki devices. An attacker can exploit this vulnerability by:
- Identifying Cisco Meraki MX or Z3 devices with AnyConnect VPN enabled and accessible from the network
- Crafting specially malformed SSL VPN connection requests with invalid or malicious parameters
- Sending these crafted requests to the target device's VPN service endpoint
- The malicious parameters trigger the validation failure, causing the VPN server process to crash
- All existing VPN connections are dropped as the server restarts
The attack requires no authentication and can be sustained to maintain persistent denial of service, preventing any remote workers from establishing VPN connectivity.
Detection Methods for CVE-2022-20933
Indicators of Compromise
- Repeated VPN server crashes and automatic restarts visible in Meraki Dashboard event logs
- Multiple simultaneous VPN disconnection events affecting all connected users
- Unusual patterns of malformed SSL/TLS handshake attempts in network traffic
- High volume of connection attempts from unexpected source IP addresses targeting VPN ports
Detection Strategies
- Monitor Meraki Dashboard for repeated "VPN server restart" or "AnyConnect service failure" events
- Implement network intrusion detection rules to identify anomalous SSL VPN connection patterns
- Configure alerting for mass VPN disconnection events that affect multiple users simultaneously
- Analyze VPN connection logs for repeated failed handshakes from specific source addresses
Monitoring Recommendations
- Enable detailed logging for the AnyConnect VPN service in Meraki Dashboard
- Set up automated alerts for VPN service availability drops or repeated restarts
- Implement rate limiting on VPN connection attempts at the network perimeter
- Monitor for sustained attack patterns by tracking VPN service uptime metrics
How to Mitigate CVE-2022-20933
Immediate Actions Required
- Apply the latest firmware updates from Cisco Meraki through the Dashboard immediately
- Review and restrict network access to VPN endpoints using firewall rules where possible
- Implement rate limiting on incoming VPN connection requests at perimeter devices
- Configure redundant VPN gateways to maintain connectivity during potential attacks
- Enable enhanced logging to detect and respond to exploitation attempts
Patch Information
Cisco Meraki has released firmware updates that address this vulnerability. The patches are distributed automatically through the Meraki cloud management platform. Administrators should verify that automatic updates are enabled or manually trigger updates through the Meraki Dashboard. For detailed patch information and affected firmware versions, refer to the Cisco Security Advisory.
Workarounds
- Restrict VPN service access to known IP ranges using firewall rules where operationally feasible
- Deploy additional security layers such as IPS systems in front of Meraki devices to filter malicious traffic
- Consider implementing geographic IP restrictions if remote workforce locations are predictable
- Establish alternative connectivity methods for business continuity during potential attacks
# Meraki Dashboard verification steps
# 1. Log into Meraki Dashboard at dashboard.meraki.com
# 2. Navigate to Organization > Firmware upgrades
# 3. Verify all MX and Z3 devices are running patched firmware versions
# 4. Enable automatic firmware updates if not already configured
# 5. Check Security & SD-WAN > Site-to-site VPN for AnyConnect settings
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
